Portolan charts are navigational maps based on
realistic descriptions of harbours and coasts. They were first made
in the 14th century in Italy,
Portugal and Spain.
With the advent of the Age
of Discovery, they were considered State secrets in Portugal
and Spain, very valuable in the description of Atlantic and Indian coastlines for newcomer English and Dutch raiding, and later trading, ships. The word
portolan comes from the Italian adjective portolano, meaning
“related to ports or harbours.” -wikipedia